Connected: An Internet Encyclopedia
RFC 1812 - E.2. Additional Pruning Rules

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### E.2. Additional Pruning Rules

Section [5.2.4.3] defined several pruning rules to use to select routes from the FIB. There are other rules that could also be used.

- OSPF Route Class
  Routing protocols that have areas or make a distinction between internal and external routes divide their routes into classes by the type of information used to calculate the route. A route is always chosen from the most preferred class unless none is available, in which case one is chosen from the second most preferred class, and so on. In OSPF, the classes (in order from most preferred to least preferred) are intra-area, inter-area, type 1 external (external routes with internal metrics), and type 2 external. As an additional wrinkle, a router is configured to know what addresses ought to be accessible using intra-area routes, and will not use inter- area or external routes to reach these destinations even when no intra-area route is available.

  More precisely, we assume that each route has a class attribute, called route.class, which is assigned by the routing protocol. The set of candidate routes is examined to determine if it contains any for which route.class = intra-area. If so, all routes except those for which route.class = intra-area are discarded. Otherwise, router checks whether the packet's destination falls within the address ranges configured for the local area. If so, the entire set of candidate routes is deleted. Otherwise, the set of candidate routes is examined to determine if it contains any for which route.class = inter- area. If so, all routes except those for which route.class = inter-area are discarded. Otherwise, the set of candidate routes is examined to determine if it contains any for which route.class = type 1 external. If so, all routes except those for which route.class = type 1 external are discarded.

- IS-IS Route Class
  IS-IS route classes work identically to OSPF's. However, the set of classes defined by Integrated IS-IS is different, such that there isn't a one-to-one mapping between IS-IS route classes and OSPF route classes. The route classes used by Integrated IS-IS are (in order from most preferred to least preferred) intra-area, inter-area, and external.

  The Integrated IS-IS internal class is equivalent to the OSPF internal class. Likewise, the Integrated IS-IS external class is equivalent to OSPF's type 2 external class. However, Integrated IS-IS does not make a distinction between inter-area routes and external routes with internal metrics - both are considered to be inter-area routes. Thus, OSPF prefers true inter-area routes over external routes with internal metrics, whereas Integrated IS-IS gives the two types of routes equal preference.

- IDPR Policy
  A specific case of Policy. The IETF's Inter-domain Policy Routing Working Group is devising a routing protocol called Inter-Domain Policy Routing (IDPR) to support true policy-based routing in the Internet. Packets with certain combinations of header attributes (such as specific combinations of source and destination addresses or special IDPR source route options) are required to use routes provided by the IDPR protocol. Thus, unlike other Policy pruning rules, IDPR Policy would have to be applied before any other pruning rules except Basic Match.

  Specifically, IDPR Policy examines the packet being forwarded to ascertain if its attributes require that it be forwarded using policy-based routes. If so, IDPR Policy deletes all routes not provided by the IDPR protocol.

Next: E.3 Some Route Lookup Algorithms

Connected: An Internet Encyclopedia
RFC 1812 - E.2. Additional Pruning Rules